Experimental studies of detonation initiation by external stimulation of exothermic reactions closely behind a propagating shock wave (SW) are reported. Gaseous and heterogeneous fuel-air mixtures have been studied. Spatially distributed electric dischargers with properly tuned triggering times are shown to provide very short distances for shock-to-detonation transition in smooth-walled tubes. The energy of each individual discharger was shown to be smaller than the critical energy required for direct detonation initiation by a single discharger. The total energy of the dischargers appeared to be lower than the critical energy of direct detonation initiation. Available experiments with deflagration-to-detonation transition (DDT) in tubes with regular or irregular obstacles are also treated as detonation initiation by a traveling ignition source. In this case, instead of external stimulation of chemical activity, the localized obstacle-induced autoignition of shock-compressed gas occurs which can be closely coupled with the propagating SW. Two possible DDT scenarios are identified, namely, 'fast' and 'slow' DDT. In case the ignition timing at obstacles is closely coupled with the SW, favorable conditions for 'fast' DDT can occur. Otherwise, the SW decouples from the ignition pulses and 'slow' DDT can occur at a later stage due to cumulating of flame-induced pressure waves and 'explosion in the explosion' phenomenon. (c) 2005 Elsevier Ltd. All rights reserved.